1. Technical Field
The present disclosure relates to a transmission rate control technique for use in transmitting a signal from a transmitter to a receiver in a wireless communication system.
2. Description of the Related Art
A 2.4-GHz wireless frequency band is often used, for example, in wireless communication represented by IEEE (Institute of Electrical and Electronics Engineers) 802.11 standards and IEEE 802.15.4 standards. The 2.4-GHz wireless frequency band is also called an ISM (Industry-Science-Medical) band.
The 2.4-GHz band is utilized in not only devices in association with wireless communication, but also in other devices, such as a microwave oven. Radiant noises from the devices, such as the microwave oven, are generated as waves interfering with a transmitter and a receiver, which perform wireless communication by employing the 2.4-GHz band. Thus, the radiant noise is one factor causing reception errors.
In an environment where the influence of the interfering wave is large, the reception errors can be suppressed by employing a transmission rate (referred to also as a “data transmission speed”) at a low speed. On the other hand, because a transmission throughput is also low at the low transmission rate, limited frequency resources cannot be effectively utilized if the low transmission rate is employed at all times.
In view of the above-described situation, there has been proposed so far a transmission rate control technique capable of adaptively selecting, from among a plurality of transmission rates, one transmission rate to be used in the wireless communication depending on the state of a transmission path, which varies with change of the ambient environment.
In the IEEE 802.11b standards, for example, four transmission rates, i.e., 0.1 Mbps, 2 Mbps, 5.5 Mbps, and 11 Mbps, are specified in accordance with the difference in modulation method. Furthermore, in the IEEE 802.11g standards, eight transmission rates, i.e., 54 Mbps, 48 Mbps, 36 Mbps, 24 Mbps, 18 Mbps, 12 Mbps, 9 Mbps and 6 Mbps, are specified in accordance with the difference in modulation method and coding rate.
The transmission rate that is determined corresponding to at least one of the modulation scheme and the coding rate, etc. in the Physical Layer is also referred to as a “PHY rate” in some cases. Furthermore, in the IEEE 802.11 standards, the transmission rate is also called an MCS (Modulation and Coding Scheme).
One example of the transmission rate control technique is disclosed in Japanese Unexamined Patent Application Publication No. 2001-103041. According to the transmission rate control technique disclosed in Japanese Unexamined Patent Application Publication No. 2001-103041, error rates are measured by performing transmission and reception of test data plural times for each of combinations of communication frequencies and data transmission speeds, and the communication frequency and the data transmission speed, which provide the lowest error rate, are selected as an optimum communication frequency and an optimum data transmission speed for use in wireless communication.
Radiant noise of a microwave over, for example, is periodic noise (see a waveform denoted by reference symbol I in FIG. 28).
Japanese Unexamined Patent Application Publication No. 9-93155 discloses a technique for, when periodic noise is generated as an interfering wave in wireless communication, reducing reception errors in consideration of periodicity of the noise. According to the technique disclosed in Japanese Unexamined Patent Application Publication No. 9-93155, the period of the interfering wave is determined. And fall and rise timings of the interfering wave are determined. And a signal is transmitted and received during a term from the fall timing to the rise timing of the interfering wave (hereinafter called a “transmission interrupt term”).
However, the above-mentioned related art has the problem that, in an environment where an interference source is present near a receiver, it takes a long time to select the transmission rate that is adapted for the environment around the receiver.